In many control and monitoring applications one or more field busses are used to establish communication between the various sensors, actuators, controllers and other components typically found at the field level in such systems. Most of these field busses are either proprietary or of limited use outside of rather narrow application areas. There has been considerable interest in using a standard communication protocol such as Ethernet™ in these applications.
Field level communications are typically established among sensors, actuators, controllers, and other devices (nodes) found in factory, industrial, environmental monitoring, agricultural and similar application environments. Unfortunately, applications in industrial settings typically have considerably different environmental, installation, and operational requirements from those found in the office environment where Ethernet™ is the dominant protocol.
First, the devices in an industrial setting are typically powered from conductors that are included in the cable carrying the signal lines. In office settings, individual nodes are powered from local 110 VAC power plugs. If a cable failure occurs in an industrial setting, the entire portion of the network serviced by that cable could be rendered inoperative. The repair of the system may be delayed for some period of time because the industrial environment has hazards that limit the repair staff to technicians who are familiar with the particular plant environment in which the cable is installed. Such failures are less frequent in the office environment since individual nodes are separately powered and repairs are not complicated by safety considerations. Hence, the network components utilized in the office implementations of Ethernet do not provide facilities for containing the failure to allow at least a portion of the network to continue functioning.
Second, office environments typically use systems in a star topology; whereas, industrial environments are better suited to a combination of star and linear topologies. In an office environment, devices are typically connected to a repeater using a star pattern. Such repeaters are normally found in ‘wiring closets’ that contain other network components such as gateways, access to wide area networks, etc. In industrial environments at the field level, two typical configurations are found. In the first configuration, there are a number of devices located within a relatively close distance. For example, a collection of devices associated with a particular machine or process such as a boiler or packaging machine could be in this configuration. In this case, a network in a star configuration is preferred. In the second configuration, there are a number of devices, or a number of collections of devices, which are relatively sparsely spaced and for which a linear topology is more natural to minimize the wiring costs. A collection of proximity sensors spaced along a conveyor line would be preferably configured in this manner.
Current hubs and 10 baseT systems are limited to star topology. The original Ethernet coaxial system was linear but currently represents a small fraction of the Ethernet installations. In an industrial environment, coax is not favored, as it is more susceptible to damage and degradation due to bending than twisted pair cables.
In principle, a linear topology can be constructed from a plurality of hubs connected together. In practice, however, the number of hubs that can be combined reliably is limited to less than 7. This limitation results from the “signal jitter” introduced by each hub. This jitter is additive; hence, as the number of hubs increases, the jitter increases until the signaling protocols fail. The maximum number of hubs, in general, is not predictable, since this number depends on the specifics of each installation. Hence, to provide a linear topology, a component that combines signals without introducing jitter is needed.
Third, individual nodes are addressed only via their node addresses in Ethernet systems. While address tables are logically sufficient to manage communications, they have a number of problems. For example, the tables are subject to installation errors and are difficult to maintain.
In addition, the information is not easily accessible by components connected to the nodes. The network topology often contains information on the functioning of devices within the network. For example, all of the devices connected to a particular hub may be related to a particular piece of machinery or group of machines that work in concert. If one machine fails, it may be necessary to send messages to the other machines to take action to contain the damage caused by the first machine's failure. This requires the data processing system at the node that detected the failure to send a message to all other devices on the relevant hub. To perform this communication, the sending processor needs to know the addresses of all devices on that hub. This information is not generally easily available to devices attached to the network. Typically only one computer in a large network will have a map of the network, and this computer, is not necessarily programmed to provide that information to a device on a node of the network. Hence, an additional level of address support is needed so that a message can be directed to all devices attached to a hub or similar device without requiring the sender to provide the address of each recipient.
Broadly, it is the object of the present invention to provide an improved network system that is better adapted to industrial environments.
It is a further object of the present invention to provide an industrial network that can function within the Ethernet standard.
It is a still further object of the present invention to provide an industrial network that supports linear topologies.
It is yet another object of the present invention to provide an industrial network that is capable of containing cable failures so that at least a portion of the network serviced by the cable can continue to function.
It is another object of the present invention to provide an industrial network in which devices can be addressed with reference to the network topology.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.